Insulating coating for filaments



R'larch 1.5, 1932 S. RUBEN INSULATING COATING FOR FILAMENTS Filed Sept. 23, 1929 I NVENTOR JIM/1 Puss/v.

Max ATTORNEY UNITED S TATES .EIGLEWOOD, xsw JERSEY, A.

PATENT OFFICE sum, 01' mrw YORK, I. Y, ASSIGNOB 'I'O RUBEN TUBE comm, OI

CORPORATION OF msmrme ooa'rme roaapplication am September is, 1929. Serial 10. mm.

This invention relates to heater type vacuum tubes, and particularly to an insulating cogetisng for the filamentary heaters of such tu One of the objects of the invention is to produce a coating for the heater of a heater type vacuum tube which is capable of withstanding a lar potential difierence between the heater an a contacti cathode without 10 leakage or electrolysis at t e normal operatingtem ratures. 1

Anot erv object of the invention is to provide a heater coating which has alow vapor pressure and high melting point, so that it 15 will not volatilize during operation of the tube in which it is used and thereby cause a change in its insulating qualities.

Another object of the invention is to provide a heater coating which may be heated to a temperature much higher t an that to be used in the normal operation of the heater to rmit complete degasification. A. her object of the invention is to (pro vide a heater coating which may be han led, bent, and stored for future use without chippinglor cracking or'otherwise deteriorating.

- Another object of the invention is to provide a itladed coating for a vacuum tube heater w ch is capable of fusing to the heator material at the point of contact but re- I sents a fine grained insulating outer sur ace.

Still anot er object of the invention is to provide a heater coating which is extremely dense and heat cond active and which is integral with the resistance material of the heat' er to give a minimum temperature gradient in the coating itself and a mirimum thermal er objects of the invention will be apparent as the descri tion thereof proceeds.

In connection wit the drawings,

a radio tube embodying my invention and 2 is a detail of one of the heater cathodeunits. e

The compounds of certain metals are valuable as insulators, and of these compounds the lower the atomic number of the metal els 0 ment the higher the specific resistance and the higher e temperature at which the in- Fi 1 is a view in elevation of a section of I sulating qualities are maintained. In other words,'compounds of barium, calcium, cadmium, strontium, and zinc lose their insulating qualities and become conductive at relar tively low temperatures while compounds of boron, magnesium, aluminum, silicon and beryllium have insulating characteristics at much higher temperatures, such temperatures varying substantially inverselyin each group tothe atomic numbers. Beryllium and magnesium having the lowest numbers, the compounds have the hi hest specific resistance and maybe heated to extremely high temperatures without decomposing or losing their insulating qualities. Beryllium is better in thisrespect than magnesium, and for use on the filaments of vacuum tubes, therefore, where high temperatures are required, I prefer to use the compounds of beryllium, although I do not wish to limit myself to this particular metal.

To attain the objects of the invention I have found it necessar to provide'a graded coating with a base ayer next to the resistance wire which will fuse to the wire at temperatures very much higher than normal operating temperatures and which will therefore act as a bonding agent between the resistance wire and an outer insulatin layer with which it is'adapted to have a ght chemical reaction so as to closely bind the two together.

' This outer layer must be highlyrefractory and of high specific resistance at the maximum operating temperature.

The inner or base layer of the coating may;-86 be made by mixing finely ground beryllium aluminosilicate (3B%Al O 6SiO, with ammonium borate I 13 0 in t e proportion by weight of three of beryllium alu-" minosilicate to one of ammonium borate with enough water to give proper consistency. The beryllium aluminosilicate should be ground fine enou h to fall thropgh a size- 250 screen mesh. is mixture is coated upon the resistance wire, which may be of tungsten or other desirable material, and acurrent of about 1.5 amperes is then passed through the wire to heat it and dry the c0at- I ing in place. Ihave found it preferable to apply two coats-of this material to form the. we

' Water the exhausting inner layer of the insulating coatin although more layers may be added if ound desirable. Two coats make the thickness approximately .01 of a millimeter.

The outer layer is preferably made of beryllium oxide (Be O The coating material for this oxide is prepared by grinding the oxide in a ball mill with 25% by weight of ammonium borate and enough distilled water to give the desired consistency. The mixture is then applied to the filament carrying the intermediate coating of beryllium aluminosilicate. This application is made cold and the coated filament then heated to about 500 C. at which temperature the ammonium borate decomposesforming ammonia gas, water vapor and boric oxide. At least five successive coatings of the beryllium oxide material should be applied. The successive heatings during the coating process also cause a partial sintering of the coating material.

The wire thus provided with its insulating coating may be handled and bent, and welded in position in a vacuum tube, the coating having enou%h flexibility so that it will not crack or chip 0 m I If the wire is not to be used at once, preferably it should receive a thin collodion coatmg so as to avoid moisture absorption by the coating. This coating may best be applied by placing the wire in a vessel containing a so-- lution of collodion and allowing the wire to remain in the solution until the occluded gas escapes, being displaced by the collodion which then forms an adherent skin layer. absorption may also be avoided by sub ecting the coating to a high temperature so as to vitrify the material thereof, but such treatment makes the coating more or less brittle and should be used preferably only when the filament is formed prior to heating.

When properly placed in the vacuum tube, the tube may be exhausted in the ordinary manner. To do this it is necessary to raise the temperature of the filament a great deal higher than it is heated in normal operation of the tube the temperature in some instances reachmg as high as 2000 C. At this temperature the first or inner layer of beryllium alummosilicate will fuse with the resistance wire and form a dense, hard, flexible coating which is substantially integral with the wire. The beryllium oxide, however, which forms the outer layer or surface of the insulating coating will withstand temperatures in the ma hborhood of 2400 an ,therefore, is not affected by the high temperature used in exhausting the tube. At temperature all physically or chemically unstable components of the coat- .mg are discharged, so that the resistance wire and coating may be completely degasified. There is, however, a slight chemical action between the inner and outer layers which C. without melting,

firmly knits the two together, maln'ng a substantially integral structure.

The drawings illustrate an application of the invention to the heater cathode of a radio tube, in which 10 is the envelope, 11 the plate, 12 the grid, 13 the cathode, and 14: the heater filament. The filament 15 is. illustrated as provided with a coating 16 which may at in tervals contact with the cathode 13. When used on a filament maintained at the proper operating temperature, this coating has been found to have no leakage or electrical conductivity when potentials up to 80 volts have been placed between thefilament and the contacting cathode, but to readily conduct heat from the filament to the cathode with a'minimum of time lag. If the tube has been evacuated the coating on the filament is absolutely stable, and as it has been completely degasified, will not cause any deterioration of the operation or efiiciency of the tube, but will protect the filament and greatly add to its life. 1

While beryllium oxide has highly desirable characteristics as an external coating and is used as a preferable material, I have found that aluminum oxide may be mixed with the beryllium oxide up to a limit of by weight of aluminum oxide, without destroying the effectiveness of the coating.

As indicated above, still other materials may be used in the insulating coating, and I do not therefore desire to limit the invention to any specific material except as defined in the appended claims.

Having thus described my invention, what I claim is:

1. A heater cathode unit for a radio tube of the heater type comprisin a refractory heater filament, a plurality o superimposed coatings containing beryllium oxide on said filament, and a tubular cathode enclosing and in approximate contact with said superimposed coatings.

2. A heater cathode unit for radio tubes of the heater type comprising a refractory heater filament, a coating containing beryllium oxide on said filament, a binder coating having a chemical aflinity for the first mentioned coating, and a cathode tube enclosing and in approximate contact with saidcoating.

3. A heater cathode unit for radio tubes of the heater type comprising a refractory heater filament, a coating of beryllium aluminosilicate on said filament, a second coating containing beryllium oxide on said first named coating, and a cathode tube enclosing and in approximate contact with said coating.

4. A heater unit for a heater t e radio tube yp comprising a refractory filament, a

, binder coating containing beryllium, a coatberyllium oxide having a chemical ty for the binder coating.

inof

-5. In a heater type radio tube, a heater cathode unit comprising a filamentary heater element, a coating on said heater containing an oxide of the ma esium group which 18 5 normally substantia y non-conductive at op erating heater temperature, and the binder coating interposed between the first mentioned coating and the heater, said binder coating, also containing an oxide of the magnesium up.

6. A eater cathode unit for a heater type 4 radio tube comprising a filamentary heater, an oxide of an element having a low atomic number which is normally substantially non- 1 conducting at normal operating temperatures of the tube llieatezi1 and l:vhicr w: r a vapor ress'ure ess an t at o tuiagsten and a bidder coatin between said oxi e and .the heater, said bin er also including an ele- 20 menthavin a low atomic number and hav in an aflimty for the said oxide.

In a radio tube of the heater type, a heater cathode unit comprising a heater fila-. ment, a coatin of beryllium oxide on said I 25 filament, and a nding coatin intermediate said coating of beryllium oxi e and the filament, the melting point of the elements oi the bonding coatin being less than that of the beryllium oxi e,- whereby an intimate,

80 dense, and highly heat conductingla er is formed intermediate the be llium oxi e and.

said, filament, and a catho e enclosing and external to said beryllium oxide coating. 1

, In testimony whereof I aflix mfi t SAMUEL EN. 

